1. Field of the Invention
The present invention relates to an image reading apparatus which reads an image of an original while scanning the original by a scanner, and particularly it relates to an image reading apparatus which detects abnormality in a position of the scanner.
2. Description of the Relates Art
In a copying apparatus using a conventional image reading apparatus, an image of an original is formed on a photoconductor drum while a scanner scans the original, and the image is transferred onto copy paper, so that the original is copied. Normally, the scanner has a predetermined initial position and it reads the original with reference to the initial position, in synchronization with supplied copy paper. Thus, in order to normally operate the copying apparatus, the scanner needs to be set always in the initial position before a start of copy operation. In order to reliably set the scanner in the initial position, a home position sensor for setting the scanner in the initial position is normally provided and a stepping motor, a DC servo motor or the like is used as a scanner motor. Hereinafter, an example of using a stepping motor will be specifically described. Particularly in a color copying apparatus of a face sequential printing system, scanning is performed for a plural number of times for respective colors and therefore it is important to register edges of the images formed by the respective scanning operations. For this reason, a stepping motor is adopted as the scanner motor. Thus, using the stepping motor, the registration is reliably performed by control of the numerical values.
However, a malfunction phenomenon might occur in the stepping motor. The malfunction phenomenon is a phenomenon that even if pulses for numerical value control are transmitted from a controller, the motor does not respond thereto. Such malfunction phenomenon occurs mainly in cases as described below.
(1) The rotation speed of the stepping motor and the load torque at that speed have a predetermined relation and if the load torque exceeds a limit in the predetermined relation, malfunction occurs. Such excess of the load torque over the limit is caused by incidents as described below (2) and (3).
(2) In case where the scanner is placed in a dusty spot, dust enters the slider of the scanner, causing malfunction.
(3) Normally the scanner moves along the rail fixed in the platen where the original is placed. If a heavy material is placed on the platen, deflection of the platen might occur. As a result, the rail of the scanner might warp, causing malfunction.
If a malfunction phenomenon as described above occurs, the position of the scanner cannot be controlled and it cannot be stopped at a predetermined stop position.
Since the copying apparatus using the conventional image reading apparatus is thus constructed, if malfunction occurs in the scanner, the scanner cannot be stopped in the predetermined position and it might collide with other components and the copying apparatus itself might be broken. In order to cope with such disadvantages, overrun sensors are conventionally provided at both ends of the moving course of the scanner so as to prevent overrun of the scanner beyond a predetermined moving range. In consequence, in the conventional copying apparatus, it is necessary to provide not only the home position sensor for setting the scanner in its initial position but also the overrun sensors for preventing malfunction.